An island is any piece of land smaller than a continent and larger than a rock that is completely surrounded by water. Very small islands — say, islands smaller than necessary for most useful purposes — are called islets. Although seldom adhered to, it is also proper to call an emergent land feature on an atoll an islet, since an atoll is a type of island.

A special type of continental island is the microcontinental island, which results when a continent is rifted. The best example is Madagascar off Africa. The Kerguelen Islands and some of the Seychelles are also examples.

Another type of volcanic island occurs where a mid-ocean rift reaches the surface. There are two examples: Iceland, which is the world's largest volcanic island, and Jan Mayen.

The last type of volcanic island are those formed over hot spots. The hot spot is stationary relative to the plate above it, so it leaves a chain of islands as the plate slowly drifts. Over long periods of time, this type of island is eventually eroded down and "drowned" by isostatic adjustment, becoming a seamount. Plate movement across a hot-spot produces a line of islands. An example is the Hawaiian Islands, from Hawaii to Kure, which extends benearth the sea surface northward as the Emperor Seamounts. Another similarly oriented chain is the Tuamotu Archipelago; its bend becomes the Line Islands. The southernmost chain is the Austral Islands, with its northern branch as most of the atolls in the nation of Tuvalu. Tristan da Cunha is an example from the Atlantic Ocean.